Oversubscription - Mitel MiVoice Office 250 Manual Book

MiVoice Office 250 Engineering Guidelines
For systems that are predominantly configured with digital telephones, a PEC-1 is not typically
required. On the other hand, systems that contain a large number of IP phones, IP trunks, IP
networking, or voice mail ports may require a PEC-1 to increase the pool of available DSP
resources.
Beginning with the v5.0 software release, the system has the ability to provide built-in Meet-Me
Conferencing functionality. This functionality is provided by the Processor Module and the
PEC-1 through DSP resources. The system can provide up to 40 conference ports with a
maximum conference size of 20 parties. The administrator can also configure the Ad Hoc
Conference Type setting under the System\Conference-Related Information folder in DB
Programming. This setting determines whether or not Ad Hoc Conferencing, Record-a-Call,
Station Monitor, and Agent Help use the Advanced DSP resources or the traditional Basic DSP
resources. When set to Advanced, the Ad Hoc Conference features share the 40 conference
ports with the Meet-Me Conference feature, thus possibly reducing the number of available
Meet-Me Conference ports; however, the Ad Hoc Conferencing party limit also increases from
four parties to twenty parties. When set to Basic, the Ad Hoc Conferencing features share 20
ports of conferencing using the traditional Basic DSP resources, and the Meet-Me Conferencing
feature has 40 ports of conferencing using the Advanced DSP resources; however, an Ad Hoc
Conference is limited to four parties.

OVERSUBSCRIPTION

Historically, PBXs provided dedicated switching hardware and wiring for every device. Although
this allowed all devices to be in use at the same time, much of the hardware was idle most of
the time. IP telephony borrows the shared resource approach of data networks to switch calls
more cost effectively. Looking back, even traditional PBXs shared things like trunks, voice mail
ports, etc., so sharing is nothing new. In fact, more efficient sharing of resources typically results
in direct cost savings. Note that a properly engineered system can be "oversubscribed" yet
never encounter contention for shared resources.
Technically, an oversubscribed system means that there are not enough resources available
for every device in the system to acquire a resource at the same time. To determine whether
or not a system is oversubscribed, we have to consider the worst-case scenario of every IP
device being on a call at the same time, as well as using other call-related features like
File-Based Music-On-Hold or T.38 Faxing. Because there are a lot of variables that go into this
calculation, determining the precise point at which a PEC-1 is required is not trivial. However,
in most scenarios, the decision can be made fairly easily.
IP phones, IP trunks, IP networking, and IP voice mail typically consume IP resources only
when they are actively on a call. Therefore, making certain that a system is not oversubscribed
may not be necessary because most sites will unlikely use or need to use all IP devices
simultaneously. However, sites that have call-critical scenarios (e.g., hospitals or call centers)
are generally not good candidates for deploying oversubscription.
Some features of the system consume DSP resources without an active call. For example, the
background music and paging features on an IP phone each use a DSP resource for the duration
of the feature; therefore, if the site uses the background music feature for company conference
calls or paging to a large number of IP phones, then the site might not be a good candidate for
oversubscription.
It is important to note that the system does not guarantee that 40 ports of conferencing are
always available. These ports are provided by the DSP resource pool; therefore, if the site
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